Due to the high strength to weight ratio, fiber reinforced composite structures have become attractive for aerospace application, such as for example, parts for airframes and propulsion power plants, and for reinforcing various type structures. Molding of such parts has been expensive, relatively time consuming and labor intensive because of the need to position elements accurately in the mold, and to carry out the process slowly to avoid porosity, air entrapment and other internal and surface defects during polymerization, cross-linking or hardening of the resin in the fiber material. Additionally, systems have required the generation of high pressures to uniformly spread the resin in the fiber.
Fiber reinforced organic resin composite structures are fabricated using two basic forms of materials, prepreg, "B" stage, resin impregnated fiber forms, and wet resin impregnation of fiber forms.
In the prepreg process, woven cloth or fabric is impregnated at one facility, with a prescribed amount of resin. The resin is staged and dried, to a "tacky" or "B" stage condition in a partially cured condition. The material is then packaged between layers of separation film and stored in containers for extended periods of time before the fabric is used and fully cured for final part processing.
The prepreg operation has a number of disadvantages. The operation requires separate manufacturing facilities, and the "B" stage material must be stored at low temperature and in sealed containers to avoid contact with moisture. The resins must be conditioned to a specific state of polymerization, and the process must be stopped to retain the "tack" condition over an extended period of time.
In the wet resin impregnation process, woven cloth or fiber is impregnated with a liquid resin that is catalyzed to process or cure in a short continuous period of time. In this process, the resin is impregnated by squeegee of ply by ply of a layup at the site of component fabrication. The impregnated material may be handled at room temperature or elevated temperature for a certain period during which the resin gels, followed by final curing either at room temperature or elevated temperature in the same tool or mold.
A wet resin impregnation process referred to as resin transfer molding (RTM) is a process to saturate the fabric with resin using two-sided tooling that is usually metal to withstand extreme pressures needed to force the resin through the fabric. The tool may be heated in order to lower the viscosity of the resin, and large presses are utilized to hold the mold together. Heat is supplied with hot oil or heating elements placed in the mold. Because of the great hydraulic pressure that is generated as the resin is pushed into the fabric to saturate the fabric, the mold will try to expand outwardly and open the internal clearances when the resin is injected into the mold. If the mold is fabricated correctly, the resin will form a wave front and move across the fabric uniformly. However, if the machining clearances vary even a few hundred thousands of an inch, the fabric will be squeezed more in one area than another and the resin will move in the path of least resistance. The areas that are compressed too much will eventually form air pockets as the resin surrounds these areas and air voids form.
Current RTM technology suffers from the unpredictability of the formation of the resin wave. Typically, aerospace grade parts using the RTM process are made with the same number of plies of fabric throughout the part. Placement of the fiber plies in the mold and machining of the mold must match precisely. If the number of fabric plies varies, the placement of ply changes must precisely match thickness changes in the mold itself. This task is difficult in a production environment.
An additional disadvantage of a wet resin process is that personnel may come in direct contact with the resin, which is undesirable. Additionally, it is difficult to create uniform resin content free of voids and bubbles. Wet resin content fabricated products are usually of higher resin content than similar prepreg fabricated products in order to ensure freedom of void within the laminant, and thus such articles are heavier than articles made from prepreg materials.
An additional form of wet resin process utilizes vacuum to draw the resin through the fabric. Resin and catalyst systems are mixed in a container, and then introduced from the container to a dry cloth fiber reinforced layer placed in a tool. A vacuum bag is placed over the dry cloth layup with an inlet tube from the resin container to an edge of the layup under the vacuum bag. The vacuum bag outlet to the vacuum source is at the center of the assembly. When a vacuum is pulled, the bag pulls against the layup, and when the resin is released, the resin passes through the tube from the resin container and impregnates the fiber reinforcement or cloth from the edge thereof. Thereafter, resin flow proceeds toward the vacuum outlet at the center of the fiber reinforcement. When the resin reaches the vacuum outlet, the article is impregnated, and the resin inlet is sealed to stop any additional resin flow. The cure cycle is completed with continued vacuum pressure and heat.
Vacuum techniques are deficient due to pressure limitations of the vacuum as well as limitations in the size of the article to be fabricated. Vacuum techniques do not satisfactorily impregnate close weave fiber reinforcement, such as carbon fiber panels, entirely along the length and width thereof, to useful large size. Vacuum techniques further have difficulty achieving low air voids.
A need has thus arisen for an improved method for manufacturing fiber reinforced composite articles in order to produce aerospace quality composite parts having high fiber volume and low air void content.
A need has arisen for a process to create composite parts having ply areas with different reinforcement. A process is needed to enable the fabrication of a part in which areas needing reinforcement can be fabricated with extra plies of fabric, to specifically address load paths in the articles, while complementing the capability of composite construction to reinforce areas of stress, or use less material where the strength of thicker layers is not required.
A need has further risen for a process of manufacturing composite articles which can be constructed with the strength of additional plies located along predetermined load paths without major changes in existing molds, which parts are fabricated to achieve consistent fiber volumes with lack of air voids throughout the final part.